1. Field of the Invention
The present invention relates to a method of manufacturing integrated circuits on semiconductor substrates. More specifically, the present invention relates to a manufacturing method which improves control of transistor drive current by varying the exposure across the wafer based on integrated circuit parameters.
2. Description of the Related Art
In a lithographic process, photoresist is applied as a thin film to a substrate and subsequently exposed through a mask. The mask contains clear and opaque features that define a pattern to be created in the photoresist layer. Areas in the photoresist layer that are exposed to the light are made either soluble or insoluble in a specific solvent called a developer. Regions of silicon oxide that are not covered by photoresist following developing are removed by etching, replicating the mask pattern in the oxide layer. The photoresist has two functions, responding to the exposing radiation to replicate the mask image in the photoresist and protecting the underlying substrate in the remaining areas of photoresist during subsequent processing.
Performance of transistors that are fabricated in an integrated circuit strongly depends on the precision of structures formed using the lithographic process and the application of various physical parameters during processing. For example, variability of gate oxide thickness, polysilicon etch bias and rapid thermal annealing (RTA) temperature all lead to variations in drive current of a transistor. Drive current, the current from source to drain of a transistor, is indicative of the speed performance of a transistor. A uniform drive current in transistors across a semiconductor wafer results in a uniform speed of transistors across the wafer and a low leakage current even in conditions when the drive current is high. On the other hand, a drive current that is nonuniform across the wafer results in a high I.sub.CC standby current, a high leakage current and high power consumption by the integrated circuit. Traditionally, drive current uniformity is sought by reducing the variability in parameters, such as gate oxide thickness, polysilicon etch bias and rapid thermal annealing (RTA) temperature, that give rise to drive current variations. However, the reduction in variability of these parameters is difficult. What is needed is a method of promoting the uniformity of drive current beyond what is achieved by reducing the variability of parameters that give rise to drive current variations.
What is also needed is an integrated circuit with a tight distribution of drive current, in which the drive current across the entire wafer is highly uniform.
What is further needed is a method of reliably producing an integrated circuit in which the drive current across an entire semiconductor wafer is uniform.